1) There is a bug in the file size reporting. I would have thought someone would have noticed that.

2) File system block size. If the reported size is the actual size on disk, and the block size surfers by a factor of 2 between the internal storage and the SD card, them the app would appear to double in size.

In some cases the entire app is not moved, seems some keep their foot in the door.
I've noticed diff apps need diff amount of themselves left in phn memory
I'm assuming for faster call up and active services the apps need to function?
*new kindle update added app2sd ,
though only about 10mb of 15 was moved, still a big saving
Guess some just cant cut the apron strings

right. the app size being reported is the portion on the phone only. so if you have an app that was 7.5 MBs and then you move in and it reports 1.5, the app didnt shrink, only 1.5MBs is now stored on the phone and the rest is on the card.